In the field of optical communication, attention has been paid to a transmission system based on the combination of a coherent detection system and digital signal processing. Such a transmission system is also called a digital coherent system, and can dramatically enhance the frequency utilization efficiency (see NPL 1, for example).
FIG. 1 is a diagram showing an optical reception device used for the digital coherent system. An optical receiver 1 converts an optical signal transmitted through an optical fiber to an electrical signal by coherent detection. A digital signal processor 2 demodulates the thus-converted electrical signal by digital signal processing. A frame processor 3 separates and processes a client signal stored in a transmission frame of the demodulated signal.
In the digital signal processor 2, an analog-to-digital (AD) converter 4 first converts an electrical signal output from the optical receiver 1 to a digital signal. Subsequently, a chromatic dispersion compensator 5 performs digital signal processing on the digital signal to estimate a chromatic dispersion amount added to the optical signal through the optical fiber as a transmission path and applies chromatic dispersion for compensating the estimated chromatic dispersion amount, thereby suppressing waveform distortion caused by the chromatic dispersion. Subsequently, an adaptive equalizer 6 adaptively compensates an output signal of the chromatic dispersion compensator 5 for waveform deterioration occurring mainly due to high-speed polarization fluctuation of the optical signal transmitted through the optical fiber. Subsequently, a demodulator 7 estimates a carrier phase from a signal waveform-equalized by the adaptive equalizer 6 and performs regeneration.
For example, a technique for extracting a clock from a demodulated signal and setting a compensated dispersion value so that the extraction sensitivity of the clock is maximum has been disclosed as a chromatic dispersion amount estimating method used in the chromatic dispersion compensator 5 (see PTL 1, for example).
The adaptive equalizer 6 comprises a finite impulse response (Finite Impulse Response, FIR) filter of N taps (N represents an integer of 2 or more) in a time direction. The high-speed waveform distortion is compensated by adaptively updating the filter coefficients of the adaptive equalizer 6 at a sufficiently higher speed than that of the polarization fluctuation of the optical signal in the optical fiber.